Boiling water nuclear reactors contain fissionable fuel which is formed in bundles of fuel rods. The fuel rod bundles are encased in a channel having a polygonal transverse cross-section, usually square. During operation of the reactor, a pressure differential develops between the interior and exterior of the fuel channel. Over time, the pressure differential, which can be quite high, causes bulging of the flat sidewalls of the fuel channel. This phenomenon is well known and is described further in U.S. Pat. No. 4,749,544 (Crowther et al.).
Many nuclear fuel channels are fabricated from two or more pieces of a zirconium alloy sheet which are bent into a desired shape and then welded together to form the channel. Processes for fabricating such channels are described in the Crowther et al. patent, as well as, U.S. Pat. No. 4,604,785 (Eddens). Such channels have two or more seam welds. Another method of fabricating fuel channels is to form a tube from the zirconium alloy sheet and then square the cross-section of the tube, for example, by passing it through a Turk's-head machine. This latter type of channel has only one seam weld and a process for making this type of fuel channel is described in U.S. Pat. No. 3,986,654 (Hart et al.).
In order to minimize longitudinal bowing of the last-described type of fuel channel, non-seam welds have been applied to one or all of the sides not containing a seam. The seam and non-seam welds are placed on the centerlines of the respective sidewalls for ease of fabrication. Experience with the known types of fuel channels that are welded in that manner has shown that after extended periods of in-reactor service, undesirable bulging of the channel sidewalls occurs. Excessive bulging of the fuel channels inhibits insertion of the control rods used to control the nuclear reaction. Accordingly, it would be highly desirable to have a fuel channel that is fabricated in such a way as to substantially reduce the amount of bulging that occurs during normal use.